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"ein" is used as an indefinite article (a / an) for masculine and neutral nouns, while "eine" is used only for femenine nouns. They change as they're used for different purposes within a sentence. Look up for "german cases" and pay special attention to the "dative case" (because "der" is used for femenine nouns there for indirect objects.)
That itself is an exception, one of many others yet to be learned. "DAS Mädchen" is a neutral, singular noun meaning "The girl", while "DIE Mädchen" is plural, meaning "The girlS". The -chen at the end is part of the noun itself, which doesn't change during the transition from singular to plural.
There is a way. Hover your mouse over the troublesome word. Duolingo will usually give you a translation. Warning: the translation isn't necessarily correct for the context in the particular sentence. In that case, all you can do is wait until you get it wrong and Duolingo gives you the right answer.